Recent work has suggested that the fMRI global signal (GS) in part may result from activity carried by the extensive sympathetic innervation of the extraparenchymal arteries of the brain. One of the pathways that potentially triggers sympathetic activity is the phenomenon of subcortical (autonomic) arousal, which is closely associated with the K-complex, a signature event observed in scalp EEG. In this work, we analyzed previously acquired data during sleep, and showed a strong association between fMRI GS, K-complexes, and photoplethysmography (PPG) from the finger skin, a proxy for sympathetic activity. Since sympathetic activity may be variable and elicited by a variety of stimuli during both sleep and wake, it likely plays an important, while largely overlooked role in most fMRI experiments.
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